Although the present invention has general applicability to any given aqueous system where the formation and deposition of scale and in particular barium sulfate is a potential problem, the invention will be discussed in detail as it concerns papermaking systems. The present invention relates to methods for inhibiting scale deposition and fouling in aqueous systems.
The water employed in papermaking systems often contain dissolved salts of barium, calcium, magnesium, etc., which can lead to scale and sludge deposits. These materials can be present in the water at its source or enter the system with the wood or pulp. In papermaking systems barium sulfate deposition has detrimental effects on papermaking, affecting paper quality, screening efficacy and production rates. Inhibitors are often employed to control barium sulfate scales. Currently, products such as polyphosphates and polyacrylates as well as polyepoxysuccinic acid (PESA) are employed as inhibitors.
In paper machines operating in an acid environment, Al.sup.+3 is present in concentrations of up to 20 mg Al.sup.+3 per liter of process water to promote paper sizing and increased paper machine drainage. The presence of Al.sup.+3 has been found to markedly reduce the efficacy of scale control products including PESA. Other metal ions that may be present as contaminants such as Fe.sup.+3 have a similar detrimental effect.
Scale can also cause rapid localized corrosion and subsequent penetration of metallic surfaces through the formation of differential oxygen concentration cells. The localized corrosion resulting from differential oxygen cells originating from deposit is commonly referred to as underdeposit corrosion.
The treatment of industrial waters to inhibit scale formation with polyepoxysuccinic acid (hereinafter PESA) is disclosed in U.S. Pat. No. 5,062,962. The general formula for PESA is: ##STR1## where n ranges from about 2 to 50, preferably 2 to 25, and M is hydrogen or a water soluble cation such as Na.sup.+, NH.sub.4.sup.+ or K.sup.+ and R is hydrogen, C 1-4 alkyl or C 1-4 substituted alkyl (preferably R is hydrogen). However, it was found that when PESA was employed in an acidic paper machine where Al.sup.+3 was present in substantial concentrations, the scale control efficacy of PESA was markedly reduced.
In addition to PESA, other scale control agents such as polyphosphates and polyacrylates were found to exhibit limitations in efficacy in such an environment. Polyphosphates function as both scale inhibitors and metal ion binders. A high dose of a polyphosphate may overcome the interference of Al.sup.+3 or Fe.sup.+3, however the cost would be prohibitive. The combination of the present invention provides a scale inhibitor which is effective at low dosages when combined with a low cost metal ion binding agent.
The scale control efficacy of PESA was also found to be decreased by the presence of acrylic acid copolymers in aqueous systems as described in application Ser. No. 071827,246 filed Jan. 28, 1992, now U.S. Pat. No. 5,248,438.